Method for making In2O3 nanoarray and use for splitting water

ABSTRACT

A method of forming a one-dimensional nanoarray of In 2 O 3  nanowires on indium foil is disclosed. The nanowires of In 2 O 3  have diameters of 30 nm-50 nm and lengths of 100 nm-200 nm, and are attached to and substantially perpendicular to the surface of the indium foil. The In 2 O 3  nanoarray may have a nanowire density of 200-300 nanowires per μm 2  indium foil and a band gap energy of 2.63-3.63 eV. The In 2 O 3  nanoarray may be formed by anodization of indium foil in an electrochemical cell subjected to a voltage of  15 - 25  V at room temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of Ser. No. 16/001,188, nowallowed, having a filing date of Jun. 6, 2018.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to an anodization method for producingone-dimensional (1-D) nanoarrays of In₂O₃.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Recently, In₂O₃ (indium(III) oxide, or indium sesquioxide) has attractedconsiderable attention as a useful material due to its promisingsemiconductor and energy harvesting applications inphoto-electrochemical systems. Various methods have been explored forthe synthesis of different In₂O₃ nanostructures, including sol-gelmethods, hydrothermal synthesis, chemical vapor deposition, and moretraditional wet chemistry techniques.

However, these methods tend to either require high energy consumption orare not able to reliably produce a regular array of nanostructures.

In view of the foregoing, one objective of the present invention is toprovide an anodization method of forming one-dimensional (1-D) In₂O₃nanoarrays on an indium foil substrate. The one-step anodization methodallows reliable synthesis of regular nanoarrays over a large area whileusing a low input of energy. The In₂O₃ nanoarrays formed by the methodmay be used in a photo-electrochemical cell or within an electroniccomponent such as a capacitor or semiconductor.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodfor producing a nanoarray of In₂O₃ nanowires on indium foil, whichinvolves subjecting an electrochemical cell to 15-25 V. Theelectrochemical cell has a working electrode comprising indium foil incontact with an electrolyte solution and a counter electrode also incontact with the electrolyte solution. In this method, nanowires ofIn₂O₃ are grown on the indium foil, forming the In₂O₃ nanoarray. In thisIn₂O₃ nanoarray, an end of each nanowire is attached to the indium foil,and the longitudinal axis of each nanowire is substantiallyperpendicular to the indium foil.

In one embodiment, the nanowires have substantially circularcross-sections.

In one embodiment, the nanowires have diameters of 30-50 nm and lengthsof 100-200 nm.

In one embodiment, the nanowires comprise In₂O₃ in a cubic crystalphase.

In one embodiment, the In₂O₃ nanoarray has a nanowire density of 200-300nanowires per μm² indium foil.

In one embodiment, the In₂O₃ nanoarray has a band gap energy of2.63-3.63 eV.

In one embodiment, the electrolyte solution has a temperature of 20-30°C.

In one embodiment, the electrolyte solution comprises polyethyleneglycol at a weight percentage of 1-8 wt % relative to a total weight ofthe electrolyte solution.

In one embodiment, the electrolyte solution comprises one or moreelectrolytes at a concentration of 0.2-1.0 M.

In one embodiment, the electrolyte solution comprises at least oneelectrolyte selected from the group consisting of chromic acid, oxalicacid, sulfuric acid, phosphoric acid, ammonium fluoride, and sodiumfluoride.

In a further embodiment, the electrolyte solution comprises sodiumfluoride.

In one embodiment, before the subjecting, the electrolyte solution issubstantially free of an aqueous form of In.

In one embodiment, the electrolyte solution is stirred during thesubjecting.

In one embodiment, the electrochemical cell further comprises areference electrode.

In one embodiment, the reference electrode is a saturated calomelelectrode.

In one embodiment, the counter electrode comprises gold, platinum, orcarbon.

In a further embodiment, the counter electrode comprises platinum.

In one embodiment, the counter electrode is in the form of a rod orwire.

In one embodiment, the electrochemical cell is subjected to a voltagethat does not vary by more than 5% of an average value throughout thesubjecting.

In one embodiment, the indium foil in contact with the electrolytesolution has a length to width ratio of 1:1-2.5:1.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is an example setup of an electrochemical cell for athree-electrode anodization of the In₂O₃ nanoarray.

FIG. 2 is an X-ray diffraction (XRD) pattern of the In₂O₃ nanoarray.

FIG. 3A is a field emission scanning electron microscope (FESEM) imageof the In₂O₃ nanoarray with a scale bar of 1 μm.

FIG. 3B is a FESEM image of the In₂O₃ nanoarray with a scale bar of 500nm.

FIG. 3C is a FESEM image of the In₂O₃ nanoarray with a scale bar of 200nm.

FIG. 3D is another FESEM image of the In₂O₃ nanoarray with a scale barof 200 nm.

FIG. 4 is an energy-dispersive X-ray (EDX) spectrum of the In₂O₃nanoarray.

FIG. 5 is a Raman spectrum of the In₂O₃ nanoarray.

FIG. 6A is an X-ray photoelectron spectroscopy (XPS) spectrum of theIn₂O₃ nanoarray showing signals from In electrons.

FIG. 6B is an XPS spectrum of the In₂O₃ nanoarray showing a signal fromO⁻² electrons.

FIG. 7A is a UV-Vis adsorption spectrum of the In₂O₃ nanoarray and agold-coated In₂O₃ nanoarray.

FIG. 7B is the spectrum from FIG. 7A calculated for band gap energies.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of“one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. Also, all values and subrangeswithin a numerical limit or range are specifically included as ifexplicitly written out.

For polygonal shapes, the term “diameter,” as used herein, and unlessotherwise specified, refers to the greatest possible distance measuredfrom a vertex of a polygon through the center of the face to the vertexon the opposite side. For a circle, an oval, and an ellipse, “diameter”refers to the greatest possible distance measured from one point on theshape through the center of the shape to a point directly across fromit.

As used herein, “compound” is intended to refer to a chemical entity,whether as a solid, liquid, or gas, and whether in a crude mixture orisolated and purified.

In addition, the present disclosure is intended to include all isotopesof atoms occurring in the present compounds and complexes. Isotopesinclude those atoms having the same atomic number but different massnumbers. By way of general example and without limitation, isotopes ofhydrogen include deuterium and tritium. Isotopes of carbon include ¹³Cand ¹⁴C. Isotopes of oxygen include ¹⁶O, ¹⁷O, ¹⁸O, and others. Isotopesof indium include, but are not limited to, ¹¹³In and ¹¹⁵In.Isotopically-labeled compounds of the disclosure can generally beprepared by conventional techniques known to those skilled in the art orby processes analogous to those described herein, using an appropriateisotopically-labeled reagent in place of the non-labeled reagentotherwise employed.

According to a first aspect, the present disclosure relates to a methodfor producing an In₂O₃ nanoarray on indium foil, which involvessubjecting an electrochemical cell to 15-25 V, preferably 17-23 V, morepreferably 18-22 V. However, in some embodiments, the electrochemicalcell may be subjected to less than 15 V or more than 25 V. Theelectrochemical cell has a working electrode comprising indium (In) foilin contact with an electrolyte solution and a counter electrode also incontact with the electrolyte solution. The counter electrode may also becalled an auxiliary electrode. In this method, nanowires of In₂O₃ aregrown on the indium foil, forming the In₂O₃ nanoarray. In this In₂O₃nanoarray, an end of each nanowire is attached to the indium foil, andthe longitudinal axis of each nanowire is substantially perpendicular tothe indium foil.

In one embodiment, the nanowires have diameters of 30-50 nm, preferably32-48 nm, more preferably 35-45 nm and lengths of 100-200 nm, preferably110-190 nm, more preferably 120-180 nm. The In₂O₃ nanowires in theseforms may be attached to or detached from the working electrode ofindium foil. Preferably the In₂O₃ nanowires are attached to the workingelectrode of indium foil. Preferably the nanowires comprise greater than90 wt %, preferably greater than 95 wt %/o, preferably greater than 99wt % In₂O₃, relative to a total weight of the nanowires. In oneembodiment, the nanowires may comprise other forms of indium oxide, suchas indium(I) oxide, indium(II) oxide, or the nanowires may comprisemetallic indium. Where the nanowires comprise compounds other thanIn₂O₃, those compounds may be present at a weight percentage of 0.001-10wt %, preferably 0.05-5 wt %, more preferably 0.1-2 wt %, relative to atotal weight of the nanowires. However, in some embodiments, thenon-In₂O₃ compounds may be present at a weight percentage of less than0.001 wt % or greater than 10 wt % relative to a total weight of thenanowires.

As defined here, nanowires are elongated shapes with diameters or widthsof 100 nm or less, preferably 80 nm or less, more preferably 60 nm orless. Here, the nanowires being “elongated” means that an aspect ratioof the longest dimension (length) of a nanowire to the next longestdimension (width) of the nanowire is 3:1-20:1, preferably 4:1-15:1, morepreferably 4:1-12:1. In some embodiments, nanowires may be lesselongated, with an aspect ratio of less than 3:1, or more elongated,with an aspect ratio of 20:1 or greater. Preferably the nanowires aresubstantially cylindrical. In this embodiment, a cross-section of ananowire has a circular or elliptical shape, or some other roundedshape. Preferably the nanowires have substantially circularcross-sections. Here, the nanowires have sides that curve in a planeperpendicular to the central axis. In other embodiments, the nanowiresmay be polygonal or prismatic, having planar sides and cross-sectionsthat are polygons, for instance, rectangular cross-sections. Inalternative embodiments, the sides of the nanowires may taper, curve, orbe angled towards a single point. The longitudinal axis of each nanowirebeing substantially perpendicular to the indium foil means that thecentral axis or length of each nanowire forms a smallest angle of60°-90°, preferably 700-90°, more preferably 75°-90°, even morepreferably 80°-90° with the surface of the indium foil, where a smallestangle of 90° is exactly perpendicular. Preferably, the lengths orcentral axes of adjacent nanowires are parallel with one another. Inother embodiments, the lengths or central axes of adjacent nanowires maybe parallel with one another while forming a smallest angle of 60°-80°,preferably 62°-75°, more preferably 630-700 with the plane of the indiumfoil surface.

In another embodiment, nanowires may be attached to the indium foil byone end, but may be bent or curved and not substantially perpendicularto the indium foil surface. For example, FIG. 3C is an FESEM image thatshows a proportion of nanowires that are bent or curved. In oneembodiment, an In₂O₃ nanoarray may comprise a mixture of nanowires thatare substantially perpendicular to the indium foil surface while othernanowires are curved or bent without being substantially perpendicularto the indium foil surface. For example, an In₂O₃ nanoarray may comprise60-90%, preferably 70-85%, of nanowires that are substantiallyperpendicular to the indium foil in relation to a total number ofnanowires, with the remaining percentage being curved or bent. In oneembodiment, nanowires with greater elongation lengths or greater lengthto width ratios may have a greater tendency of being curved or bent. Inanother embodiment, nanowires may be curved or bent in such a way that aline connecting their base to their distal end is substantiallyperpendicular, while a side of the nanowire is not substantiallyperpendicular.

In one embodiment, the nanowires may be hollow, similar to boxes ornanotubes. Preferably, however, the nanowires are solid. In someembodiments, the nanowires may be considered nanowhiskers, nanorods,nanospikes, nanostrings, nanofilaments, nanopicks, nanopegs, nanopins,nanostakes, nanopillars, nanoprisms, nanofibers, nanotubes, ornanocylinders. In other embodiments, two or more nanowires side by sidemay merge together towards the indium foil surface.

The ends of the nanowires may have tips in a variety of configurations.Where the nanowires are attached to the indium foil, these tips refer tothe unattached ends of the nanowires, or the ends distal to the indiumfoil. In one embodiment, the nanowires may have flat tips that areeither perpendicular to the sides of the nanowires (as in a rightcylinder) or tilted at an angle relative to the sides (as in a truncatedcylinder). In another embodiment, the nanowires may have curved tips,such as hemispherical or dome-shaped tips. In one embodiment, thenanowires have conical tips, with curved edges leading towards a bluntpoint. This shape may be similar to the pointed end of an ellipsoid oran egg. FIGS. 3B-3D show example images of nanowires with generally flattips. Preferably, the nanowires are substantially rotationally symmetricaround their central axis, as shown in FIGS. 3B and 3D.

Preferably, in one embodiment, the nanowires have a uniform shape.Alternatively, the shape may be non-uniform. As used herein, the term“uniform” refers to an average consistent shape that differs by no morethan 10%, by no more than 5%, by no more than 4%, by no more than 3%, byno more than 2%, by no more than 1% of the distribution of nanowireshaving a different shape. As used herein, the term “non-uniform” refersto an average consistent shape that differs by more than 10% of thedistribution of nanowires having a different shape. In one embodiment,the shape is uniform and at least 90% of the nanowires are substantiallycylindrical, and less than 10% are polygonal or substantially prismatic.In another embodiment, the shape is non-uniform and less than 90% of thenanowires are substantially cylindrical, and greater than 10% arepolygonal or substantially prismatic. In one embodiment, the nanowiresof the present disclosure are monodisperse, having a coefficient ofvariation or relative standard deviation, expressed as a percentage anddefined as the ratio of the particle size standard deviation (a) to theparticle size mean (p) multiplied by 100, of less than 25%, preferablyless than 10%, preferably less than 8%, preferably less than 6%,preferably less than 5%, preferably less than 4%, preferably less than3%, preferably less than 2%. In a preferred embodiment, the nanowires ofthe present disclosure are monodisperse having a size distributionranging from 80% of the average nanowire size to 120% of the averagenanowire size, preferably 90-110%, preferably 95-105% of the averagenanowire size.

Nanoparticle characterization may be used to establish understanding andcontrol of the In₂O₃ nanoarray anodization and application. In oneembodiment, it is envisioned that characterization is done using avariety of techniques. Exemplary techniques include, but are not limitedto, electron microscopy (TEM, SEM), atomic force microscopy (AFM),ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering(DLS), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF),powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy(EDX), thermogravimetric analysis (TGA), Fourier transform infraredspectroscopy (FTIR), matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry (MALDI-TOF), Rutherford backscatteringspectrometry (RBS), dual polarization interferometry, time-of-flightsecondary ion mass spectrometry (ToF-SIMS), electron energy lossspectroscopy (EELS), high-angle annular dark field (HAADF), nearinfrared (NIR) spectroscopy, nuclear magnetic resonance (NMR), orcombinations thereof.

In one embodiment, the In₂O₃ nanoarray has a nanowire density of200-300, preferably 220-280, more preferably 230-270, even morepreferably 235-265 nanowires per μm² indium foil. In alternativeembodiments, the nanowire density may be smaller than 200 or greaterthan 300 nanowires per μm² indium foil. In one embodiment, the nanowiresare attached to the indium foil in a substantially regular array,wherein any 1 μm² region of the indium foil comprises 85%-99%,preferably 87-98%, more preferably 90-98% nanowires in a close-packedarray, and a packing density that does not vary by more than 10%,preferably by no more than 7% of the bulk average nanowire density.

In one embodiment, the nanowires are discrete and separated byinter-nanowire gaps or inter-nanowire distances of 1-50 nm, preferably3-45 nm, preferably 5-40 nm, preferably 6-38 nm, preferably 7-35 nm,preferably 8-32 nm, most preferably 9-30 nm. The inter-nanowire distancerefers to the shortest distance between the outer edges of twoneighboring nanowires. In a preferred embodiment, the nanowires of thepresent disclosure have an average surface to surface inter-nanowiredistance of less than 200% of their average diameter, preferably lessthan 150% of their average diameter, preferably less than 100% of theiraverage diameter, preferably less than 50% of their average diameter,preferably less than 25% of their average diameter, preferably less than10% of their average diameter, preferably less than 5% of their averagediameter. In one embodiment, the nanowires may have tapered or curvedtips, and may be touching or merging with one another (that is, aninter-nanowire gap of 0 nm) where they attach to the indium foil. In oneembodiment, the inter-nanowire gaps may be in the form of pores ordepressions where indium of the indium foil was removed during theanodization process. These pores or depressions may have diameterssimilar to the lengths of the inter-nanowire distances listed above. Thepores or depressions may have a maximum depth of 20-500 nm, preferably30-400 nm, more preferably 40-380 nm, even more preferably 45 nm-350 nmbelow that of the original plane of the indium foil surface. The poresor depressions may have a greatest width or diameter of 20 nm-1 μm,preferably 30-500 nm, more preferably 35-450 nm, even more preferably37-420 nm. In one embodiment, the pores may be shaped with lengths,diameters, and/or edges similar to the geometry of nanowires, describedpreviously, being inverted into the indium foil.

In one embodiment, inter-nanowire gaps may be elongated, leading to theformation of grooves or channels between nanowires. These grooves orchannels may have widths of 10-25 nm, preferably 12-20 nm, and lengthsof 90-400 nm, preferably 100-350 nm. An uninterrupted wall of a channelmay be formed by 3-12, preferably 4-8 adjacent nanowires. In oneembodiment, grooves or channels may be longer or more common with anarray having a higher packing regularity and/or a shorter length ofnanowires. In one embodiment, a row of nanowires may be merged togethercloser to the indium foil, forming a shape similar to a picket fence.

In one embodiment, the nanowires comprise In₂O₃ in a cubic crystalphase. For example, the In₂O₃ may be in a cubic crystal phase similar tothe crystal phase of bixbyite ((Mn,Fe)₂O₃). The cubic crystal phase ofIn₂O₃ may have 16 molecules (i.e. molecular weights) of In₂O₃ per unitcell and an 1 a 3 space group symmetry. The density of crystal phaseIn₂O₃ may be 6.8-7.3 g/cm³, preferably 7.0-7.2 g/cm³. In alternativeembodiments, the In₂O₃ may be in a different crystal phase or may be inan amorphous phase, for example, the In₂O₃ may be in a rhombohedralphase. In one embodiment, the In₂O₃ of a single nanowire may be in amixture of crystal and amorphous phases. For example, a single nanowiremay have 2-20 wt %, preferably 3-15 wt % In₂O₃ in an amorphous phaserelative to a total weight of the nanowire, with cubic crystal phaseIn₂O₃ as the remaining phase. Preferably, the crystal phase may bedetermined by analyzing X-ray diffraction patterns.

In one embodiment, the In₂O₃ nanoarray has a band gap energy of2.63-3.63 eV, preferably 2.83-3.43 eV, more preferably 3.03-3.23 eV.However, in alternative embodiments, the In₂O₃ nanoarray may have a bandgap energy of less than 2.63 eV or greater than 3.63 eV. The band gapenergy may be influenced by the density of the nanowires, theirgeometry, their dimensions, their regularity, and/or the type of indiumfoil substrate. In one embodiment, a person having ordinary skill in theart may be able to adjust the band gap energy to different values bychanging those characteristics of the In₂O₃ nanoarray. In oneembodiment, the band gap energy may be determined from a UV-Visabsorption spectrum, as shown in FIGS. 7A and 7B.

In one embodiment, the anodization method may be used to make In₂O₃nanoarrays having dimensions different than those mentioned above. Forinstance, In₂O₃ nanowires may be produced that have diameters smallerthan 30 nm or greater than 50 nm, and/or lengths smaller than 100 nm orgreater than 200 nm. In a related embodiment, the anodization method maybe used to make an In₂O₃ nanoarray having a nanowire density of lessthan 220 or greater than 300 nanowires per μm² indium foil.

In a related alternative embodiment, the method may be used to makeIn₂O₃ in the form of nanostructures that are not nanowires. Forinstance, the In₂O₃ may be made in the form of nanoparticles,nanosheets, nanoplatelets, nanocrystals, nanospheres, nanorectangles,nanotriangles, nanopentagons, nanohexagons, nanoprisms, nanodisks,nanocubes, nanoribbons, nanorods, nanobeads, nanotoroids, nanodiscs,nanolaminas, nanoshells, nanogranules, nanowhiskers, nanoflakes,nanofoils, nanopowders, nanoboxes, nanostars, tetrapods, nanobelts,nano-urchins, nanofloweres, etc. and mixtures thereof.

In some embodiments, the electrochemical cell may be subjected to thepreviously mentioned voltages for 1-3 hours, preferably 1.5-2.5 hours,more preferably 1.8-2.2 hours, or about 2 hours. In other embodiments,however, the electrochemical cell may be subjected to those voltages fora time shorter than 1 hour or longer than 3 hours. In alternativeembodiments, the subjecting may use a lower voltage for a longer time,or a higher voltage for a shorter time, in order to achieve similarresults.

This electrochemical method as discussed above may be considered ananodization process for producing the In₂O₃ nanoarray. As defined here,anodization is an electrolytic passivation process used to increase thethickness of a metal oxide layer on the surface of a metal. Here, forexample, the metal is indium, and the metal oxide is indium oxide(In₂O₃).

During anodization of a metal, the metal oxide layer is grown by passinga direct current through an electrolytic solution, with the metal ofinterest serving as the anode (the positive electrode). The electricalcurrent releases hydrogen at the cathode (the negative electrode) andoxygen at the surface of the anode, creating a build-up of the metaloxide.

Anodization is usually performed in an acidic electrolyte solution,which slowly dissolves the formed metal oxide, counteracting itsdeposition. However, in some embodiments a basic electrolyte solutionmay be used. This dual action of deposition and dissolution leads to thecreation of nanostructures, such as nanopores and nanoparticles. Ingeneral, the metal oxide is both grown down into the metal surface andout from the metal surface by similar thicknesses. In addition, if ametal is anodized on all sides, where the metal is of the same type andin contact with the electrolyte solution, then all linear dimensionswill increase by approximately the same layer thickness.

The nanopores may have diameters of 5-200 nm, preferably 10-150 nm, morepreferably 12-50 nm, and these nanopores allow the electrolyte solutionand current to reach the un-reacted metal and continue growing the metaloxide layer to greater thicknesses beyond those produced by justautopassivation. However, in some embodiments, following the anodizationprocess, these nanopores may permit air or water to reach un-reactedmetal and initiate corrosion if not sealed. Thus, for certain anodizedmetals, a protective dye and/or corrosion inhibitor may be applied. Insome embodiments, crevices, cracks, or depleted regions larger than thenanopores discussed above may be formed during the anodization process.

Several parameters of an anodization process may be modified to lead todifferent sizes and morphologies of nanostructures. These parametersinclude, but are not limited to, electrolyte type and concentration, pH,solution temperature, current, voltage, stirring rate, electrode surfacearea, electrode type, and exposure time. A variable DC current may beapplied at a fixed voltage, or a fixed DC current may be applied at avariable voltage. In some instances, AC current or pulsed current may beused. A person having ordinary skill in the art may be able to adjustthese and other parameters, to achieve different desired nanostructures.

The working electrode may be considered the anode, and the counterelectrode may be considered the cathode. Thus, in one embodiment, theworking electrode is the positive electrode, and the counter electrodeis the negative electrode. However, in an alternative embodiment wherethe electrochemical cell is subjected to alternating current (AC), orthe polarity of the electrochemical cell is otherwise switched, theworking electrode may momentarily become the negative electrode whilethe counter electrode momentarily becomes the positive electrode.

In one embodiment, the electrolyte solution has a temperature of 20-30°C., preferably 22-28° C., more preferably 22-27° C., or about roomtemperature. In another embodiment, the electrochemical cell is operatedwhile using a cooling mechanism, such as a water bath, ice packs, cooledair, or tubing containing a flowing coolant. In other embodiments, theelectrolyte solution may have a temperature lower than 20° C. or greaterthan 30° C., and in a further embodiment, the electrochemicalanodization to form the nanowires may occur successfully at thosetemperatures. Preferably the electrochemical cell is maintained atatmospheric pressures, however, in alternative embodiments, the pressuremay be decreased below or increased above atmospheric pressures.

Preferably, to maintain uniform concentrations and/or temperatures ofthe electrolyte solution, the electrolyte solution may be stirred oragitated during the step of the subjecting. The stirring or agitatingmay be done intermittently or continuously. This stirring or agitatingmay be by a magnetic stir bar, a stirring rod, an impeller, a shakingplatform, a pump, a sonicator, a gas bubbler, or some other device.Preferably the stirring is done by an impeller or a magnetic stir bar.

In one embodiment, the electrochemical cell is a vessel having aninternal cavity for holding the electrolyte solution. The vessel may becylindrical, cuboid, frustoconical, spherical, or some other shape. Thevessel walls may comprise a material including, but not limited to,glass, polypropylene, polyvinyl chloride, polyethylene, and/orpolytetrafluoroethylene, and the vessel walls may have a thickness of0.1-3 cm, preferably 0.1-2 cm, more preferably 0.2-1.5 cm. The internalcavity may have a volume of 2 mL-100 mL, preferably 2.5 mL-50 mL, morepreferably 3 mL-20 mL. In another embodiment, for instance, for smallscale or benchtop anodization, the internal cavity may have a volume of100 mL-50 L, preferably 1 L-20 L, more preferably 2 L-10 L. In anotherembodiment, for instance, for pilot plant anodization, the internalcavity may have a volume of 50 L-10,000 L, preferably 70 L-1,000 L, morepreferably 80 L-2,000 L. In another embodiment, for instance, forindustrial plant-scale anodization, the internal cavity may have avolume of 10,000 L-500,000 L, preferably 20,000 L-400,000 L, morepreferably 40,000 L-100,000 L. In one embodiment, one or moreelectrochemical cells may be connected to each other in parallel and/orin series. In another embodiment, the electrolyte solution may be incontact with more than one working electrode and/or more than onecounter electrode.

In one embodiment, the electrolyte solution comprises at least oneelectrolyte selected from the group consisting of chromic acid, oxalicacid, sulfuric acid, phosphoric acid, ammonium fluoride, and sodiumfluoride. Preferably, the electrolyte solution comprises sodiumfluoride. In one embodiment, the electrolyte solution comprises one ormore electrolytes at a concentration of 0.2-1.0 M, preferably 0.3-0.8 M,more preferably 0.4-0.6 M. However, in some embodiments, the electrolyteconcentration may be less than 0.2 M or greater than 1.0 M. In anotherembodiment, the electrolyte solution may comprise two or moreelectrolytes at different ratios to one another. For instance, a molarratio of a first electrolyte to a second electrolyte may be 1:10-10:1,preferably 1:5-5:1, more preferably 1:2-2:1, or, in one embodiment,about 1:1. In other embodiments, the electrolyte solution may comprisetrichloroacetic acid, gluconic acid, formic acid, perchloric acid,hydrochloric acid, nitric acid, ammonium sulfate, ammonium chloride,sodium chloride, potassium chloride, toluenesulfonic acid,benzenesulfonic acid and/or tartaric acid at a concentration of 0.01-1M, preferably 0.05-0.8 M, more preferably 0.1-0.7 M. In one embodiment,the electrolyte solution may have a pH of 1.0-7.0, preferably 1.2-6.0,more preferably 1.5-5.0. In another embodiment, the electrolyte solutionmay have a pH of 7.0-13.0, preferably 7.8-11.0, more preferably8.5-10.0, though in some embodiments, the pH may be less than 1.0 orgreater than 13.0. In one embodiment, the subjecting may be carried outstarting with one type of electrolyte solution, and then changed to adifferent type of electrolyte solution.

In one embodiment, before the subjecting, the electrolyte solution issubstantially free of an aqueous form of indium, meaning that theelectrolyte solution contains 10 ppb or less, preferably 5 ppb or less,more preferably 1 ppb or less of indium. In this embodiment, theelectrolyte solution is substantially free of a dispersion of indium ordissolved indium in the form of atomic In, In(I), In(II), In(III) or acompound containing In. Here, the only indium present in the entiresystem before subjecting the electric potential is the indium of theindium foil. In other words, the nanowires are formed only from indiumthat originates from the indium foil. However, in an alternativeembodiment, aqueous In(I), In(II), and/or In(III) may be present in theelectrolyte solution before the subjecting, for example, by mixing anindium salt such as InCl₃. In another alternative embodiment, colloidalindium or indium nanoparticles may be added to the electrolyte solutionbefore the subjecting. In these alternative embodiments, where In(I),In(II), and/or In(III) may be present in the electrolyte solution beforethe subjecting, the electrochemical process that grows the In₂O₃nanoarray may not necessarily be anodization, but some otherelectrodeposition process.

In one embodiment, the electrolyte solution does not contain asurfactant. However, in alternative embodiments, a surfactant may bepresent to direct the anodization and growth of the nanowires, thusinfluencing their geometry.

Exemplary ionic surfactants include, but are not limited to, (1) anionic(based on sulfate, sulfonate or carboxylate anions), for example,perfluorooctanoate (PFOA or PFO), perfluorooctanesulfonate (PFOS),sodium dodecyl sulfate (SDS), ammonium lauryl sulfate, and other alkylsulfate salts, sodium laureth sulfate (also known as sodium lauryl ethersulfate (SLES)), alkyl benzene sulfonate, soaps, and fatty acid salts;(2) cationic (based on quaternary ammonium cations), for example, cetyltrimethylammonium bromide (CTAB) (also known as hexadecyl trimethylammonium bromide), and other alkyltrimethylammonium salts,cetylpyridinium chloride (CPC), polyethoxylated tallow amine (POEA),benzalkonium chloride (BAC), and benzethonium chloride (BZT); and (3)zwitterionic (amphoteric), for example, dodecyl betaine, cocamidopropylbetaine, and coco ampho glycinate.

Exemplary nonionic surfactants include, but are not limited to, alkylpoly(ethylene oxide), alkylphenol poly(ethylene oxide), copolymers ofpoly(ethylene oxide) and poly(propylene oxide) (commercially known asPoloxamers or Poloxamines), polyoxyethylene octyl phenyl ether (TRITONX-100®), alkyl polyglucosides, for example, octyl glucoside and decylmaltoside, fatty alcohols, for example, cetyl alcohol and oleyl alcohol,cocamide MEA, cocamide DEA, and polysorbates (commercially known asTWEEN 20, TWEEN 80), for example, dodecyl dimethylamine oxide.

Exemplary biological surfactants include, but are not limited to,micellular-forming surfactants or surfactants that form micelles insolution, for example, DNA, vesicles, phospholipids, and combinationsthereof. In other embodiments, other additives may be used to direct thegrowth of the nanowires, such as polyethylene glycol or sodium citrate.In one embodiment, the electrolyte solution comprises polyethyleneglycol at a weight percentage of 1-8 wt %, preferably 2-7 wt %, morepreferably 3-6 wt % relative to a total weight of the electrolytesolution. However, in some embodiments, the electrolyte solution maycomprise polyethylene glycol at a weight percentage less than 1 wt % orgreater than 8 wt % relative to a total weight of the electrolytesolution. The polyethylene glycol may have a weight average molecularweight of 0.2-500 kDa, preferably 1-300 kDa, more preferably 2-100 kDa.

In a preferred embodiment, the electrolyte solution comprises both0.3-0.8 M sodium fluoride and 1-8 wt % polyethylene glycol relative to atotal weight of the electrolyte solution.

By incorporating at least one surfactant in the electrolyte, thenanowires or other nanostructures may become ordered, for example, byself-assembly. A surfactant or additive such as those listed above maybe present in the electrolyte solution at a concentration of 1-500 mM,preferably 10-400 mM, more preferably 50-350 mM.

In one embodiment, the indium foil in contact with the electrolytesolution has a length to width ratio of 1:1-2.5:1, preferably 1:1-2:1,more preferably 1:1-1.5:1, though in some embodiments, the indium foilmay have a length to width ratio of greater than 2.5:1.

Preferably, in these embodiments, the indium foil is square orrectangular, though other shapes with curved and/or straight edges arepossible. In one embodiment, the indium foil may have a length orlongest dimension of 0.4-10 cm, preferably 0.5-5 cm, more preferably0.8-1.2 cm. In one embodiment, the indium foil may have a width of0.1-1.5 cm, preferably 0.2-1.0 cm, more preferably 0.3-0.6 cm. In oneembodiment, the indium foil may have a thickness of 0.001-4.00 mm,preferably 0.1-1.0 mm, more preferably 0.2-0.8 mm. However, in someembodiments, the indium foil may have a longest dimension greater than10 cm or smaller than 0.4 cm, a width greater than 1.5 cm or smallerthan 0.1 cm, and/or a thickness smaller than 0.001 mm or greater than4.00 mm. The indium foil may be substantially planar, or may be curvedor rolled into a tube or scroll. Preferably the indium foil issubstantially planar. In alternative embodiments, the indium foil may bein a different form, such as a rod, wire, ring, disc, bead, mesh,perforated sheet, multi-layered foil, or some other shape. In oneembodiment, the indium foil may comprise an outer layer of indiumattached to a layer of a different material. For instance, the indiumfoil may comprise a layer of indium deposited on a glass coverslip.

In one embodiment, the working electrode in contact with the electrolytesolution consists of indium foil and no other metal. However, in someembodiments, the indium foil may not comprise 100% indium, but maycomprise 3 wt % or less, 2 wt % or less, preferably 1 wt % or less ofimpurities, such as other metals, metal oxides, and adsorbed organicmaterials. In alternative embodiments, the working electrode may be analloy comprising indium, such as indium tin oxide (ITO), an alloy ofindium and chromium, or an alloy of indium and zinc.

In an alternative embodiment, other metals may be used in theanodization method to create nanostructures comprising different metaloxides. These metals may be aluminum, titanium, magnesium, zinc,niobium, tantalum, cadmium, tin, gallium, thallium, lead, antimony,tungsten, zirconium, ruthenium, barium, selenium, bismuth, alloysthereof, or some other metal.

In one embodiment, the working electrode may be perforated, scratched,polished, etched, or modified in some other way before the subjecting.In another embodiment, the working electrode may be cleaned with asolvent such as, for example, acetone or ethanol and/or a detergent suchas an industrial detergent of TFD4 type. This cleaning may be carriedout in an ultrasonication bath. In other embodiments, cleaning mayinvolve UV irradiation or exposure to argon plasma and/or oxygen plasma.These cleaning steps may be used to avoid contamination by externalagents such as organic pollutants.

During the anodization process, the current density drawn on the workingelectrode surface may be 1-1,000 mA/cm², preferably 5-600 mA/cm², morepreferably 10-300 mA/cm², however, in some embodiments, the currentdensity may be less than 1 mA/cm² or greater than 1,000 mA/cm². In otherembodiments, the voltage or the exposed surface area of the electrodemay be adjusted in order to achieve a certain current density.

In an alternative embodiment, the In₂O₃ nanoarray may be formed bylithography, more preferably nanolithography. Nanolithography techniquesmay be categorized as in series or parallel, mask ormaskless/direct-write, top-down or bottom-up, beam or tip-based,resist-based or resist-less methods all of which are acceptable in termsof the present disclosure. Exemplary nanolithography techniques include,but are not limited to, optical lithography, photolithography, directedself-assembly, extreme ultraviolet lithography, electron beamlithography, electron beam direct write lithography, multiple electronbeam lithography, nanoimprint lithography, step-and-flash imprintlithography, multiphoton lithography, scanning probe lithography,dip-pen nanolithography, thermochemical nanolithography, thermalscanning probe lithography, local oxidation nanolithography, molecularself-assembly, stencil lithography, X-ray lithography, laser printing ofsingle nanoparticles, magnetolithography, nanosphere lithography, protonbeam writing, charged particle lithography, ion projection lithography,electron projection lithography, neutral particle lithography andmixtures thereof. In another alternative embodiment, the In₂O₃ nanoarraymay be formed by a sol-gel or chemical vapor deposition method. Inanother alternative embodiment, the In₂O₃ nanoarray may be synthesizedby two or more techniques, for instance, a nanolithography method andthen an electrodeposition method.

In one embodiment, the counter electrode comprises gold, platinum, orcarbon. In a further embodiment, the counter electrode comprisesplatinum. In one embodiment, the counter electrode and/or the workingelectrode may be in the form of a wire, a rod, a cylinder, a tube, ascroll, a sheet, a piece of foil, a woven mesh, a perforated sheet, or abrush. The counter electrode and/or the working electrode may bepolished in order to reduce surface roughness or may be texturized withgrooves, channels, divots, microstructures, or nanostructures. Similarto what was mentioned for the indium foil, the counter electrode maycomprise impurities, an alloy comprising In, or other metals favorableto the anodization method.

In another further embodiment, where the counter electrode comprisesplatinum, the counter electrode is in the form of rod or wire.Alternatively, the counter electrode may comprise some otherelectrically-conductive material such as platinum-iridium alloy,iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloyand/or some other electrically-conductive material, where an“electrically-conductive material” as defined here is a substance withan electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C. Inanother alternative embodiment, the working electrode may not compriseindium, but may comprise any of the previously mentioned metals.

In a preferred embodiment, the counter electrode has at least one outersurface comprising an essentially inert, electrically conductingchemical substance, such as platinum, gold, or carbon. In anotherembodiment, the counter electrode may comprise solid platinum, gold, orcarbon. The form of the counter electrode may be generally relevant onlyin that it needs to supply sufficient current to the electrolytesolution to support the anodization current required for the formationof the In₂O₃ nanoarray in the anodization process. The material of thecounter electrode should thus be sufficiently inert to withstand thechemical conditions in the electrolyte solution, such as acidic or basicpH values, without substantially degrading during the anodizationprocess. The counter electrode preferably should not leach out anychemical substance that interferes with the anodization process or mightlead to undesirable contamination of the In₂O₃ nanoarray formed duringanodization.

In a further embodiment, where the counter electrode comprises platinum,the counter electrode may be in the form of a mesh. In one embodiment,the counter electrode in the form of a mesh may have a nominal apertureor pore diameter of 0.05-0.6 mm, preferably 0.1-0.5 mm, more preferably0.2-0.4 mm, and/or a wire diameter of 0.01-0.5 mm, preferably 0.08-0.4mm, more preferably 0.1-0.3 mm. In other embodiments, the counterelectrode may be considered a gauze with a mesh number of 40-200,preferably 45-150, more preferably 50-100. In other embodiments, thecounter electrode may be in the form of a perforated sheet or a sponge.In one embodiment, the counter electrode may be in the form of a meshwith one or more bulk dimensions (length, width, or thickness) aspreviously described for the indium foil working electrode. In a furtherembodiment, the counter electrode may be in the form of a mesh withsimilar a length and width as described for the indium foil workingelectrode.

In one embodiment, the counter electrode is in the form of a rod orwire. The rod or wire may have straight sides and a circularcross-section, similar to a cylinder. A ratio of the length of the rodor wire to its width may be 1,500:1-1:1, preferably 500:1-2:1, morepreferably 300:1-3:1, even more preferably 200:1-4:1. The length of therod or wire may be 0.5-50 cm, preferably 1-30 cm, more preferably 3-20cm, and a long wire may be coiled or bent into a shape that allows theentire wire to fit into an electrochemical cell. The diameter of the rodor wire may be 0.5-20 mm, preferably 0.8-8 mm, more preferably 1-3 mm.In some embodiments, a rod may have an elongated cross-section, similarto a ribbon or strip of metal.

In other embodiments, the electrochemical cell comprises a referenceelectrode as a third electrode. A reference electrode is an electrodewhich has a stable and well-known electrode potential. The highstability of the electrode potential is usually reached by employing aredox system with constant (buffered or saturated) concentrations ofeach relevant species of the redox reaction. A reference electrode mayenable a potentiostat to deliver a stable voltage to the workingelectrode or the counter electrode. The reference electrode may be astandard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), areversible hydrogen electrode (RHE), a saturated calomel electrode(SCE), a copper-copper(II) sulfate electrode (CSE), a silver chlorideelectrode, a pH-electrode, a palladium-hydrogen electrode, a dynamichydrogen electrode (DHE), a mercury-mercurous sulfate electrode, or someother type of electrode. In a preferred embodiment, a referenceelectrode is present and is a saturated calomel electrode (SCE).However, in some embodiments, the electrochemical cell does not comprisea third electrode.

In one embodiment, the potential may be applied to the electrodes by abattery, such as a battery comprising one or more electrochemical cellsof alkaline, lithium, lithium-ion, nickel-cadmium, nickel metal hydride,zinc-air, silver oxide, and/or carbon-zinc. In another embodiment, thepotential may be applied through a potentiostat or some other source ofdirect current, such as a photovoltaic cell. In one embodiment, apotentiostat may be powered by an AC adaptor, which is plugged into astandard building or home electric utility line. In one embodiment, thepotentiostat may connect with a reference electrode in the electrolytesolution. Preferably the potentiostat is able to supply a relativelystable voltage. For example, in one embodiment, the electrochemical cellis subjected to a voltage that does not vary by more than 5%, preferablyby no more than 3%, preferably by no more than 1.5% of an average valuethroughout the subjecting. In another embodiment, the voltage may bemodulated, such as being increased or decreased linearly, being appliedas pulses, or being applied with an alternating current.

In one embodiment, the In₂O₃ nanoarray produced by the first aspect ofthe disclosure may be used to receive light within aphoto-electrochemical cell. A photo-electrochemical cell may be assimple as an electrochemical cell that has a transparent window to allowelectromagnetic radiation to reach the electrode of interest.Alternatively, a photo-electrochemical cell may be enclosed in a flat,transparent housing so that the photo-active electrode has a greatersurface area facing an irradiation source. A photo-electrochemical cellmay be used with mirrors, lenses, shutters, optical filters, opticalfibers, or other optical devices in order to modulate or direct anincoming electromagnetic radiation. The incoming electromagneticradiation may have a wavelength of 200-700 nm, preferably 250-600 nm,more preferably 300-500 nm, even more preferably 360-420 nm. Preferablythe incoming electromagnetic radiation comprises one or more wavelengthsthat have an energy equivalent or greater than the band gap energy ofthe In₂O₃ nanoarray. For example, the incoming electromagnetic radiationmay have one or more wavelengths of 10-395 nm, preferably 150-300 nm,more preferably 200-250 nm, where wavelengths in the range of 10-400 nmmay be considered UV radiation. In one embodiment, thephoto-electrochemical cell includes water in contact with the In₂O₃nanoarray. In a further embodiment, the water may be present in anaqueous electrolyte solution.

In an alternative embodiment, the In₂O₃ nanoarray may be used in thefield of batteries, fuel cells, hydrogen sensors, semiconductors (suchas field effect transistors), magnetic semiconductors, capacitors, datastorage devices, biosensors (such as redox protein sensors),photovoltaics, liquid crystal screens, plasma screens, touch screens,OLEDs, antistatic deposits, optical coatings, reflective coverings,anti-reflection coatings, and/or reaction catalysis. Similarly, in oneembodiment, the In₂O₃ nanoarray may be coated with another material. Forexample, the In₂O₃ nanoarray may be coated with a layer of gold. Agold-coated In₂O₃ nanoarray may then be used for analyte detection usingsurface enhanced Raman scattering (SERS).

In another embodiment, In₂O₃ nanoarray produced by the method of thefirst aspect may be used to decompose water into H₂ and O₂, whichinvolves irradiating a photo-electrochemical cell as described abovewith an electromagnetic radiation having a wavelength of 200-700 nm,preferably 210-600 nm, more preferably 220-500 nm, even more preferably250-420 nm, where the photo-electrochemical cell comprises water incontact with the In₂O₃ nanoarray. Preferably the source of theelectromagnetic radiation is sunlight, though in some embodiments, theirradiation source may be a flame, a lantern, a gas discharge lamp (suchas a xenon, sodium, or mercury vapor lamp), an incandescent bulb, alaser, a fluorescent lamp, an electric arc, a light emitting diode(LED), a cathode ray tube, or some other source of light.

The examples below are intended to further illustrate protocols forpreparing, characterizing the In₂O₃ nanoarray, and uses thereof, and arenot intended to limit the scope of the claims.

Example 1

Synthesis of Indium(III) oxide (In₂O₃) Nanoarrays By Anodization Method

Provided herein is a homogeneous, In₂O₃ nanoarray formed on a large areawith uniform coverage. This In₂O₃ nanoarray is fabricated by a simplelow voltage and room temperature anodization method. By fine-tuning thereaction parameters of the anodization method, the morphology of theIn₂O₃ nanoarray may be precisely tailored.

The main aspect of this invention is the one step fabrication method ofan In₂O₃ nanoarray on an indium foil substrate via anodization/etchingin an electrolytic solution. The current passes between two electrodesin contact with an electrolyte solution. The voltage and current may bechanged according to the resulting surface anodization. Additionalfeatures of the invention will become ostensible from the followingdescription.

Features of the invention are now described with reference to FIGS. 1 to6B. These figures describe, using common reference numerals, componentsand other features of the In₂O₃ nanoarray, the electrochemical cell, andmethods of use.

FIG. 1 shows the anodization or electrochemical cell 8 of the presentinvention. The electrochemical cell 8 comprises the electrodes 2, 3, 4,electrolyte solution 7, and magnetic stirrer 5, which are all held in anon-conducting, non-reactive vessel.

For the operation of the electrochemical cell 8 for the anodization ofthe indium foil and creation of the In₂O₃ nanoarray, the potentiostat 1applies a constant voltage of 20 V across the working electrode 3 andthe counter electrode 4 at room temperature, using a saturated calomelelectrode (SCE) 2 as a reference electrode. Indium (In) foil (0.5 cm×1cm) serves as the working electrode 3. A platinum (Pt) wire electrode isused as a counter electrode 4. A magnetic stir bar 5 is rotated with thehelp of a magnetic stir plate 6 to stir the electrolyte solution 7,keeping the concentration homogeneous throughout the course of theanodization method. Here, the electrolyte solution 7 comprises 0.5 M NaFand 5% polyethylene glycol in water and is in contact with all threeelectrodes 2, 3, 4. The stir plate is not heated.

FIG. 2 is an X-ray diffraction (XRD) pattern of the In₂O₃ nanoarray,used to determine the crystallinity and phase purity of the In₂O₃. Thepeaks were indexed and compared with a database. A characteristic (222)peak at 2θ=30° indicates the formation of cubic In₂O₃, as a (222)lattice is the dominant facet of the formed product. This peak was usedto calculate the average In₂O₃ nanowire thickness of 30 nm, by employingthe Debye-Scherrer equation [Patterson, A. L. Phys Rev 56 (1939)978-incorporated herein by reference in its entirety].

Other characteristic XRD peaks are observed at 2θ=23°, 38°, 44°, 47.5°,53°, and 63°. These peaks can be indexed, respectively, to the (211),(411), (332), (431), (611), and (622) crystal planes of the cubiccrystal In₂O₃. The XRD peaks are matched with JCPDS card #06-0416.

FIGS. 3A and 3B show field emission scanning electron microscope (FESEM)images of In₂O₃ nanoarrays, with scale bars of 1 μm and 500 nm,respectively. FIGS. 3C and 3D also show FESEM images of nanoarrays, bothimages with scale bars of 200 nm. These micrographs clearly indicate theuniform one dimensional (1D) growth of the nanowires over the surface ofthe metallic indium substrate. The thickness of an individual nanowireis about 30-40 nm. Such small-sized nanowires can be helpful in variousapplications due to their large surface area to volume ratios.

FIG. 4 is an energy-dispersive X-ray (EDX) spectrum of the In₂O₃nanoarray, which shows peaks related to indium and oxygen only. Thesmaller peaks at 3-4 keV are related to the metal coating used for FESEMobservations.

FIG. 5 is a Raman spectrum of In₂O₃ nanoarray. The Raman spectrum ofIn₂O₃ has been reported to show characteristic peaks in the lowervibrational region of 50 to 700 cm⁻¹, so it is widely known that thecubic crystal structure of In₂O₃ relates to the I³ _(a), T¹ _(h) spacegroup. For such space groups the most Raman active symmetries are A_(g),E_(g), and T_(g), while the remaining T_(a) vibrations are IR active.The In₂O₃ Raman spectrum shows the expected phonons at 79 cm⁻¹ (medium),129.5 cm⁻¹ (very strong), 189 cm⁻¹ (weak), 210 cm⁻¹ (weak), and 307 cm⁻¹(strong), followed by weak peaks at 363.5 cm⁻¹, 413 cm⁻¹, 493 cm⁻¹, anda medium peak at 628 cm⁻¹. The presence of intense phonons in the Ramanspectra of In₂O₃ nanowires is associated with the excitation of theA_(g), E_(g), and T_(g) vibrational modes.

FIGS. 6A and 6B are X-ray photoelectron spectroscopy (XPS) spectra ofthe In₂O₃ nanoarray. The spectrum in FIG. 6A shows the characteristicdoublets of In at 3d_(3/2) and 3d_(5/2) in the energy range of 443-455eV due to spin orbital splitting. The spectrum in FIG. 6B shows thecharacteristic single peak for oxygen, which may be attributed to theoxygen anion of the indium oxide.

FIG. 7A shows a UV-Vis absorption spectrum of pure In₂O₃ and gold-coatedIn₂O₃. FIG. 7B is a band gap calculated spectrum being used to determineband gap energies. The UV-Vis results of FIG. 7A show strong absorbanceof pure In₂O₃ below 300 nm, while the band gap energy was found at 3.13eV in FIG. 7B.

The above experimental description discloses a single step method forthe electrochemical synthesis of In₂O₃ nanoarrays on a large area of anindium substrate and with a high reproducibility. The method uses anindium metal substrate as the working electrode, and electrochemicalreactions produce a growth of In₂O₃ nanoarrays from this substrate. Themethod uses a low voltage to produce highly uniform arrays of In₂O₃nanostructures, which may be used in electronic components.

The invention claimed is:
 1. A method for making an In₂O₃ nanoarrayphoto-electrochemical cell and splitting water to form H₂ and O₂,comprising: subjecting an electrochemical cell to 15-25 V to anodize anindium foil, the electrochemical cell comprising: a working electrodecomprising indium foil in contact with an electrolyte solution, areference electrode, and a counter electrode in contact with theelectrolyte solution, wherein nanowires of In₂O₃ are grown on the indiumfoil, forming the In₂O₃ nanoarray, wherein an end of each nanowire isattached to the indium foil, and wherein the longitudinal axis of eachnanowire is substantially perpendicular to the indium foil, then placingthe In₂O₃ nanoarray in a photo-electrochemical cell to make thephoto-electrochemical cell and adding water; and irradiating thephoto-electrochemical cell containing the water and the In₂O₃ nanoarrayto form H₂ and O₂ by splitting the water.
 2. The method of claim 1,wherein the In₂O₃ nanoarray has a band gap energy of 2.63-3.63 eV. 3.The method of claim 1, wherein the electrolyte solution has atemperature of 20-30° C.
 4. The method of claim 1, wherein theelectrolyte solution comprises polyethylene glycol at a weightpercentage of 1-8 wt % relative to a total weight of the electrolytesolution.
 5. The method of claim 1, wherein the electrolyte solutioncomprises one or more electrolytes at a concentration of 0.2-1.0 M. 6.The method of claim 1, wherein the electrolyte solution comprises atleast one electrolyte selected from the group consisting of chromicacid, oxalic acid, sulfuric acid, phosphoric acid, ammonium fluoride,and sodium fluoride.
 7. The method of claim 1, wherein before thesubjecting, the electrolyte solution is substantially free of an aqueousform of In.
 8. The method of claim 1, wherein the electrochemical cellis subjected to a voltage that does not vary by more than 5% of anaverage value throughout the subjecting.
 9. The method of claim 1,wherein the indium foil in contact with the electrolyte solution has alength to width ratio of 1:1-2.5:1.